Crankshaft damper?

blue72s · 01-01-2005, 01:16 PM

I have just been reading Paul Frere's book - 911 Story, and the quote on pg 188 & 189:
"The largest bore that could be achieved with Biral cylinders was 87.5mm, due to their comparatively thick walls, which with a 66mm stroke limited the capacity to 2,380cc - 120cc short of the 2,500cc class limit. Only when, with the introduction of the 2.4 litre E-series cars, the stroke was increased to from 66mm to 70.4mm, did it become possible to raise the racing engine's capacity up to the class limit by using cylinders with an 86.7mm bore. The power output was raised to 275 bhp at 7,900 rpm. Unfortunately this engine was rather troublesome: crankshaft vibrations tended to loosen the flywheel securing bolts, and in several instances the crank itself broke. As this sort of problem had never cropped up with a 66mm stroke crankshaft, a few examples of the 2.5 litre engine were made using a 66mm stroke with cylinders of 89mm, implying that Nikasil cylinders had to be used. Obviously, however, a solution had to be found if the engine was to be further enlarged in future years.

Pg191:
(In 1973) The Carrera RS 2.7 became the base for a racing version which immediately outclassed everything that had been developed before. Running as a Group 4 GT car, full throttle let more than 300 bhp loose. A remedy for the crankshaft breakages was found in the use of a damper at the rear end, but there were still some difficulties with the flywheel occasionally coming loose."

So, what is this damper at the rear end?
TIA.

911pcars · 01-01-2005, 04:57 PM

A crankshaft damper is usually a two-piece affair, often doubling as a pully, separated by a band of thick rubber bonded between the two parts. The outer metal ring provides some mass that, along with the rubber, dampens shaft vibration at a critical rpm range. Frere's statement implies Porsche used a crank pulley-type damper and it wasn't altogether successful as the vibration still managed to loosen the flywheel bolts. However, I've never seen anything like it (as though that means anything). Maybe that's why Porsche eventually went from 6 flywheel bolts to 9 bolts on the later engines (but that was in '78).

Let's hope sharper minds can provide an answer.

Sherwood

TimT · 01-01-2005, 05:39 PM

Arent flat six engines inherently "in balance"?

911pcars · 01-01-2005, 08:41 PM

Arent flat six engines inherently "in balance"?"

Yes, but blu72S quoted from Frere:

< (In 1973) The Carrera RS 2.7 became the base for a racing version which immediately outclassed everything that had been developed before. Running as a Group 4 GT car, full throttle let more than 300 bhp loose. A remedy for the crankshaft breakages was found in the use of a damper at the rear end, but there were still some difficulties with the flywheel occasionally coming loose.">>

In order to generate that much power from a 2.7, one would have to spin the crank fairly high (+9000 rpm). Typically, the 2.7 RS engines produced 210 HP @ 6300 rpm. - not close to 300 hp. Maybe the words, "became the base" provides a hint. According to B. Anderson (911 Performance Handbook), the RSR engine, which produced 308, 315 or 330 HP depending on the induction system, used a 70.4mm crank with larger radius fillets on the corners of the bearing journals to provide more strength (prevent cracking). Maybe the fillets were the final solution to the crank breakage problem. Not sure about the flywheel.

Sherwood

blue72s · 05:28 AM

Quote:

Originally posted by 911pcars
According to B. Anderson (911 Performance Handbook), the RSR engine used a 70.4mm crank with larger radius fillets on the corners of the bearing journals to provide more strength (prevent cracking). Maybe the fillets were the final solution to the crank breakage problem.

Yes you're right - it's on pg 68 (and pg185). However, I would like to see a photo as I still don't quite understand. Anyone?

If one builds - say a 2.7RS engine for street use - should he put this damper or as in BA's words 'large radius fillets' for added insurance?

Quicksilver · 01-02-2005, 06:00 AM

On the street it would nearly impossible to get the sustained high RPM operation that would lead to cracking. Unless the crank was a real hand grenade, it would require sustained operation at a resonate RPM to get the crank to start cracking. You get this when you are wound out in high gear. The RPMs don't change quickly in high gear so when you pass a resonate speed the resonation can amplify. Boom, damage, open wallet.

The boxer six is in balance but we aren't talking about balance here, this is about torsional resonance. Steel always acts as a spring. That is one of its properties. (modulus of elasticity) When the piston pushes down the crank will take a small torsional twist as it transmits power to the end of the crank. The problem arises when the frequency of the power application coincides with natural resonate frequency of the crank. Then the energy acts like a child on a swing. A little push each time at the correct moment and the back and forth action gets bigger and bigger instead of having a chance to dissipate.

Give the stored energy a chance to build until it exceeds the limits of the steel and then you have a problem. The RPM is the problem, not the power output.

The ultimate problem is cracking so another way to avoid the problem is to make the crank more crack resistant. A basic precept of this is, "A crack has to start somewhere". Increasing the size of the crank fillets spreads the concentrated stress to avoid a stress level that will cause damage. As an easy example: Straighten a clothes hanger and grab it at each end. Now start bending it back and forth through a 90° range. You will grow old before it breaks. Now get a 1" bar of steel made of the exact same steel as a clothes hanger and turn it on a lathe so there is a narrow groove at one spot that reduces its diameter to the size of a coat hanger. You have basically just reinforced a coat hanger everywhere except at one spot. That is your "stress riser". You grab it at each end and bend it a couple times and it breaks. Smoothly changing the size and spreading the area where it bends is the secret to toughness.

Another way to increase the resistance to cracking is surface treatments. If you can keep the surface from allowing a crack to start you can amazingly increase the resistance to cracking. The common techniques are to cause the surface to become compressed. That way when you start bending the surface will become relaxed instead of under tension. Shot peening is a way of physically compressing the surface. Nitriding is a way of increasing the size of the molecule chemically plus it adds some additional benefits for the bearing surface.

The 911's crank is very stiff and has a higher natural frequency then the common V8s so you don't see the need for a dampener like you do for a Ford or Chevy. Also the later, larger engines don't rev as high so they avoid some of this problem to begin with.

Wayne

mpdevelopment · 01-02-2005, 06:52 AM

The crankshaft used in the 2.4-3.0 liter carrera is actually not very stiff. when the crank was changed from 66mm to 70.4mm stroke the overlap between the rod and main journals was reduced and the webs were thinned in order to increase the rod bearing width. This resulted in a crankshaft with reduced torsional and bending stiffness. It was quite common for a 2.8liter RSR to shed its flywheel. This is a torsional problem compounded by the very small flywheel bolt circle. The enlarged fillets increase the fatigue strength of the crank in bending and torsion. Increased bending strength is needed due to the movement of the mag. crankcase at higher power and speed. I have never seen the damper spoken of in these books.

Quicksilver · 01-02-2005, 10:41 AM

One of the coolest engineering solutions that I have seen related to this was on the 917. They found that with the length of the crank, there would be one harmonic RPM in the engine's rev range. It wasn't a strong enough harmonic to damage the crank but they were worried about feeding that harmonic into the cam drive.

In this harmonic there was a 'dead spot' in the middle of the crank where the harmonic wouldn't effect it; kind of a fulcrum in the center you might say. That is where the hooked up the cam drive. A very simple, minimalist solution.

Wayne

Bill Verburg · Bill Verburg's Garage

Quote:

One of the coolest engineering solutions that I have seen related to this was on the 917

Quite correct most 4cyl motorcycle engines do the same thing.

A flat 6 is inherently balanced but that doesn't mean that the crank doesn't whip. A reason to go back to aluminum c/c was to better control this tendancy.

The only production engine to use a harmonic balancer was the 964. The h/b was incorporated in the crank pulley.

The other method of controlling crank harmonics was the deletion of the fully counterweighted cranks on some Ts(I forget what years)

The problem w/ fly wheel bolts loosening first arose w/ the 2.5 they also broke cranks. The cause was the long stroke(70.4 vs 66mm). The result was to go back to 66mm stroke and use Nickasilcyl w/ an 89mm bore. This config gave the same hp and torque w/o the fragility.

It's interesting to see that they went back to the 70.4mm stroke w/ the 2.4 & 2/7s w/o any issues(though I have seen 2.7 cranks snap when over revved).